At 12:17 PM 7/8/97 -0400, Mark Shelly wrote:
>Thomas R. Holtz, Jr. wrote:
>>Archaeopteryx is exceedingly primitive, and the common ancestor of
>>dromaeosaurids and birds probably looked like something very similar
>>to Archaeopteryx.
>
> As such, early bird ancestors and related groups probably still retained
>the theropod socketed teeth, tail, and hand claws. In other words, they
>were flying theropods.
Well, even hummingbirds are flying theropods. Very derived flying
theropods, but theropods nonetheless.
>It seems highly probable that many of these
>flying theropods flew to isolated islands and reverted to the predatory
>bipedal predator niche that their theropod features allowed, becoming
>flightless, but still retaining some of the flight adaptations.
Very likely, but we do not as yet have evidence that this did indeed occur.
> With the primary locomotion means being flight in this ancestral group,
>the need for tail based Caudi-femoralis running muscles would have been
>reduced, possibly resulting in the rear facing pubic bones (for balance)
>and thinner, stiffer tails.
There are other biomechanical explanations for this reorganization of the
pelvic/hindlimb/tail muscle complex, which may have been exapted into the
flight tail of birds.
> Since not all flying theropod groups would have adapted the same weight
>reducing changes in the same sequence, there may be many theropod groups
>such as the velociraptors or ostrich mimics that do not look similar to each
>other but evolved from flying theropod dinosaurs.
Although this is possible, the data that would be necessary for me to accept
this hypothesis is a phylogenetic analysis placing one or more of the
typical "non-avian" theropods within a clade with known flying members.
This has not yet been the result of theropod phylogenetic analyses.
> If modern birds had not given up their teeth and hand claws during their
>evolution, the predominant predator might still be very theropod-like.
At various points in the Cenozoic, large predatory flightless bitds did
become the local dominant predators.
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:th81@umail.umd.edu
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661